Well this might sound redundant and not to say you haven't tried this but is the battery charged and the Variax modeling light on? Are you using the quarter inch output or the VDI?

Are you getting any sound at all from the mag pickups? I find this to be strange since the mags should work no matter what. This sounds like it could be something wrong with the quarter inch out or if you are using the vdi something internal with the power.

So, just for the sake of completeness, I put the battery in the guitar and plugged it into an amp via the 1/4" cable. As expected, the guitar sounded fine through the mag pickups, both with and without battery.

After doing that for a few minutes, I decided to go back over to the HD500/DT25 combo, connecting to the HD500 the VDI cable--no 1/4". To my surpise, the little blue LED backlighting the selector knob lit up, and the guitar worked fine once again. I then updated the firmware to 2.0 with no problems. Tried 5 or 6 power cycles of the guitar, and each time it came back up ok.

Is it possible that playing through the 1/4" jack did something? Now I'm worried that I may just have an unstable guitar that works only some of the time--although, I'm happy for now since it works once again!